1. Field of the Invention
The present invention relates to a liquid crystal display device. And, the invention relates to a liquid crystal display device which can make gradational display. The invention particularly relates to a liquid crystal display device where each pixel is provided with a memory element for storing a data signal applied.
2. Description of the Related Art
Liquid crystal displays are thin and low electric power consumers and being used extensively for notebook computers and other electrical equipments. One of remarkable features of the liquid crystal display device is its particularly low power consumption as compared with other display devices such as CRTs and plasma displays, and its future application to portable information-processing equipment is expected.
Such portable equipment is desired to have a display which consumes low power of 500 mW or below, and preferably several mW. In response to such a demand, a reflective type liquid crystal display device has been used conventionally, because it is a simple matrix type using a TN (twisted nematic) liquid crystal, does not need a back light, and consumes only low electric power. But, this display has a disadvantage that since the TN type liquid crystal needs a polarizer, its reflectance is about 30% and dark. Besides, the simple matrix type has a disadvantage that when the number of pixels is increased, contrast is lowered, and display image quality is deteriorated. Therefore, using a PCGH (phase change guest host type) mode liquid crystal not requiring to use a polarizer and driving by active matrices, there has been an attempt to produce a display provided with a high reflectance and a high contrast.
FIG. 7 shows one example of the circuit structure of a pixel in a conventional liquid crystal display device. This circuit structure of the pixel shown in FIG. 7 is the same as a conventional transmission active matrix liquid crystal display device. When a thin film transistor (TFT) 93 is turned on by a scan signal applied to a gate line 92, a data signal voltage applied to a signal line 91 is applied to a liquid crystal layer 94. And, an electrical charge is applied to a storage capacitance 96 by a storage capacitance line (Cs line) 95. As it is known well, it is generally necessary to apply an alternating voltage to the liquid crystal layer 94, and the data signal voltage, which becomes positive or negative with a voltage of an opposed electrode 97 formed on an opposed substrate at the center, is applied to the signal line 91 to drive the pixel.
Such a liquid crystal display device needs the application of an AC voltage to the liquid crystal layer even when the displaying image does not change at all. Therefore, a pixel potential is renewed whenever selected by frame cycling. Power consumption P when an alternating voltage is applied to the capacitance is expressed as follows: EQU P=f.times.V.sup.2 .times.C
where, f denotes a frequency, V a voltage, and C capacitance. Therefore, the higher the frequency, the higher the voltage or the higher the capacitance, the higher the power consumption becomes.
For the AC drive of the liquid crystal display device, the drive frequency of each pixel is a frame frequency, the drive frequency of a signal line is the product of a frame frequency and the number of scanning lines, the drive frequency of a signal line driver IC is the product of the number of all pixels on the screen and a frame frequency, and, if a division drive is performed, it is a value obtained by further dividing by a division number. For example, in a diagonally 10.4-inch color VGA (640.times.RGB.times.480 pixels) liquid crystal display device, a signal line IC has power consumption of about 1 W. Therefore, an A4-size 150 dpi-equivalent high definition liquid crystal display device has the number of pixels of about 1600.times.1200 6.25 times greater than VGA, resulting in a disadvantage that power consumption is as high as 2 to 3 W or more. When a high power-consuming liquid crystal display device is used for any portable information-processing equipment, there are disadvantages that a battery exhausts soon and the usable duration is shortened.
On the other hand, it is known that power consumption can be reduced by using a bistable ferroelectric liquid crystal (SSFLC: Surface Stabilized Ferroelectric Liquid Crystal). Since the ferroelectric liquid crystal has a memory, the voltage supply can be stopped unless the screen is changed. But, the bistable ferroelectric liquid crystal has a disadvantage that its orientation is disturbed by an impact, resulting in a screen failure. Therefore, it cannot be used for a portable display device. Besides, the liquid crystal having memory often has its contrast and reflectance limited, and its display has a problem in quality. For example, the SSFLC in the display mode needs a polarizing plate and the screen is dark with the reflectance of about 30%. Besides, since what is displayed by the SSFLC is basically limited to a binary display because it is bistable, its presentation capacity (namely, a volume of information) is lowered to great extent as compared with the display mode which can make gradation display. This constitutes a very serious problem in making a color display. When spatial light modulation is made by a dither method or the like to make the gradation display, effective resolution is degraded. Besides, when temporal modulation is made by frame rate control or the like, a flicker takes place, thus the SSFLC cannot be applied to animations.
As described above, the displays for a personal computer and portable information-processing equipment show mostly still images, and an alternating voltage is supplied to a signal line even when the screen is not changed, resulting in waste of electric power.
In view of the circumstances, the present invention aims to remedy the above-described disadvantages and to provide a liquid crystal display device which does not consume much electric power.
And, the invention aims to achieve high resolution with a simplified pixel circuit.
New display devices have been developed to take the place of the conventional CRTs in recent years. Among such display devices, liquid crystal displays which are thin and do not consume much power are being used broadly for OA equipment.
The liquid crystal display devices are classified into a simple matrix type and an active matrix type using an active element in view of the differences in their driving methods. Main liquid crystal display devices are of the active matrix type excelling in display performance.
FIG. 11 is a diagram showing one example of the circuit structure of a pixel in a conventional active matrix liquid crystal display device. Each pixel is provided with, for example, a thin film transistor 1 as a switching element. The thin film transistor 1 is turned on or off by a scan signal applied to a scanning line 3. When the pixel is selected, namely when the thin film transistor 1 is on, a data signal applied to a signal line 4 is applied to a liquid crystal layer 2 through a source and drain of the thin film transistor 1. When the pixel is not selected, the state of the liquid crystal is held by the capacitance of the liquid crystal layer 2 itself and a storage capacitance 5. But, the change in the state of pixel with time due to the movement of an electric charge in the storage capacitance 5 and in the capacitance of the liquid crystal (C.sub.LC) can not be avoided. Accordingly, the data signal applied to the pixel is refreshed in a cycle of about 1/60 seconds.
Even when the data signal applied to the pixel does not change while displaying, for example, a still image, the above conventional liquid crystal display device needs the data signal applied to the pixel refreshed frequently. This is a big barrier against the reduction of power consumption. Portable information-processing equipment and OA equipment often display a still image, consuming excess power. Particularly, when the portable information-processing equipment is a high power consumer, there is a disadvantage that its usable duration is shortened.
To remedy such a disadvantage, a method of using a bistable ferroelectric liquid crystal (SSFLC) has been proposed as described above. But, it has disadvantages in operation stability and display quality. For example, its orientation is disturbed by an impact, resulting in a screen failure. Therefore, it cannot be used as a portable display device. Besides, since the selection is limited to on and off of light, spatial modulation is required for halftone display, and resolution is degraded.
Japanese Patent Laid-Open Application No. Hei 5-119298 discloses an example of providing each pixel with an image information storage function by incorporating a ferroelectric substance into each pixel as a memory element. An equivalent circuit diagram of a pixel of this liquid crystal display device is shown in FIG. 18A. In this circuit, an AC voltage is applied to an opposed electrode, and according to the state of a ferroelectric layer 8, a thin film transistor 1 which has the ferroelectric layer 8 connected to a gate electrode is turned on or off. As a result, a terminal A has a predetermined potential or a floating state and can select two states as an AC voltage to be applied to a liquid crystal layer 2. However, even this example cannot display halftone. It needs to adopt spatial modulation to display halftone, but display quality is deteriorated. Besides, the terminal A falls in a floating state, and its stable operation is deteriorated.
On the other hand, Japanese Patent Laid-Open Application No. Hei 3-89391 discloses an example of a liquid crystal display device using a thin film transistor having memory. An equivalent circuit diagram of a pixel of this liquid crystal display device is shown in FIG. 18B. In this example, a voltage divided by a thin film transistor 1 and a thin film transistor having memory with image information written is outputted to a terminal A, and this output voltage is applied to a liquid crystal layer 2. This liquid crystal display device can control gradation in an analog fashion, but the voltage to be applied to the liquid crystal layer 2 includes a DC component. It is known that the liquid crystal layer 2 is deteriorated by the presence of the DC component. Thus, its display is questionable in view of reliability.
As described above, the conventional active matrix liquid crystal display device showing a still image needs to renew the image frequently and has a disadvantage of consuming much power. Besides, the conventional liquid crystal display device using a ferroelectric liquid crystal or a ferroelectric substance is technically difficult to store a halftone data signal or to make display without degrading reliability. And, it also has a disadvantage of lowering the display resolution.
In view of the above-described disadvantages, an object of the present invention is to provide a liquid crystal display device which can store halftone image information (data signal) into each pixel and which has high image quality and high reliability without consuming much power.